Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and Related Group 13 Nucleophiles with Alkyl Halides

Effective Alkyl-Alkyl Cross-Coupling with an Iron-Xantphos Catalyst: Mechanistic and Structural Insights

While iron-catalyzed C(sp2)-C(sp3) cross-couplings have been widely studied and developed in the last decade, alkyl-alkyl cross-coupling systems with iron remain underdeveloped despite the importance of C(sp3)-C(sp3) bonds in organic synthesis. A major challenge to the development of these reactions is the current lack of fundamental insight into ligand effects and organoiron intermediates that enable effective alkyl-alkyl couplings. The current study addresses this longstanding limitation using a combination of 57Fe Mössbauer spectroscopy, SC-XRD (single-crystal X-ray diffraction) and reactivity studies of alkyl-alkyl coupling with iron-Xantphos to define the in situ formed iron-Xantphos intermediates in catalysis. Combined with detailed reactivity studies, the nature of the key mechanistic pathways in catalysis and ligands effects to achieve effective alkyl-alkyl cross-coupling over competing β-H elimination pathways are probed. Overall, these foundational studies provide a platform for future bespoke ligand and pre-catalyst design for alkyl-alkyl cross-coupling methods development with sustainable iron catalysis.

A Rationally Designed Iron(II) Catalyst for C(sp3)-C(sp2) and C(sp3)-C(sp3) Suzuki-Miyaura Cross-Coupling

Despite the paramount importance of the Suzuki-Miyaura coupling (SMC) in academia and industry, and the great promise of iron to offer sustainable catalysis, iron-catalyzed SMC involving sp3-hybridized partners is still in its infancy. We herein report the development of a versatile, well-defined electron-deficient anilido-aldimine iron(II) catalyst. This catalyst effectively performed C(sp3)-C(sp2) and C(sp3)-C(sp3) SMC of alkyl halide electrophiles and (hetero)aryl boronic ester and alkyl borane nucleophiles respectively, in the presence of a lithium amide base. These couplings operated under mild reaction conditions and displayed wide functional group compatibility including various medicinally relevant N-, O- and S-based heterocycles. They also tolerated primary, secondary and tertiary alkyl halides (Br, Cl, I), electron-neutral, -rich and -poor boronic esters and primary and secondary alkyl boranes. Our methodology could be directly and efficiently applied to synthesize key intermediates relevant to pharmaceuticals and a potential drug candidate. For C(sp3)-C(sp2) couplings, radical probe experiments militated in favor of a carbon-centered radical derived from the electrophile. At the same time, reactions run with a pre-formed activated boron nucleophile coupled to competition experiments supported the involvement of neutral, rather than an anionic, (hetero)aryl boronic ester in the key transmetalation step.

Exploring the Reactivity of B-Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH2 and tBuPH2 react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H2 C2 B10 H9 -9-PHR (R=2,4,6-Me3 C6 H2 (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BF(C6 F5 )2 (2 a, b) through nucleophilic para substitution of a C6 F5 ring followed by fluoride transfer to boron. Further reaction with Me2 SiHCl prompted a H-F exchange yielding the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BH(C6 F5 )2 (3 a, b). The reaction of 2 a, b with one equivalent of R'MgBr (R'=Me, Ph) gave the extremely water-sensitive frustrated Lewis pairs 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )B(C6 F5 )2 (4 a, b). Hydrolysis of the B-C6 F4 bond in 4 a, b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BX(C6 F5 )2 ]- [DMSOH]+ (R=Mes, X=F (6 a), R=tBu, X=F (6 b); R=Mes, X=H (7 a), R=tBu, X=H (7 b)). Reaction of 2 a, b with an excess of Grignard reagents resulted in the addition of R' at the boron atom yielding the anions [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BR'(C6 F5 )2 ]- (R=Mes, R'=Me (8 a), R=tBu, R'=Me (8 b); R=Mes, R'=Ph (9 a), R=tBu, R'=Ph (9 b)) with [MgBr(Et2 O)n ]+ as counterion. The ability of the zwitterionic compounds 3 a, b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.

A new protocol for synthesizing diarylmethanes using a benzyltitanium reagent as a nucleophile

The first palladium-catalyzed cross-coupling of various substituted benzyltitaniums with aryl triflates is presented for the synthesis of diarylmethanes in yields of up to 94% through highly selective C–O bond functionalization. The benzyltitaniums act as nucleophiles to realize the C(sp2)–C(sp3) cross-coupling with high efficiency in short reaction times. The reactions proceed at 60°C and show excellent functional groups tolerance.

Merging Halogen-Atom Transfer (XAT) and Copper Catalysis for the Modular Suzuki–Miyaura-Type Cross-Coupling of Alkyl Iodides and Organoborons

We report here a mechanistically distinct approach to achieve Suzuki–Miyaura-type cross-couplings between alkyl iodides and aryl organoborons. This process requires a copper catalyst but, in contrast with previous approaches based on palladium and nickel systems, does not utilizes the metal for the activation of the alkyl electrophile. Instead, this strategy exploits the halogen-atom-transfer ability of α-aminoalkyl radicals to convert secondary alkyl iodides into the corresponding alkyl radicals that then are coupled with aryl, vinyl, alkynyl, benzyl, and allyl boronate species. These novel coupling reactions feature a simple setup and conditions (1 h at room temperature) and facilitate access to privileged motifs targeted by the pharmaceutical sector.

Iron-catalysed alkene and heteroarene H/D exchange by reversible protonation of iron-hydride intermediates

The iron-catalysed C(sp2)–H bond H/D exchange reaction using CD3OD is reported for both heterocycles and alkenes. Characterisation of the key C–H metallation intermediates provided evidence for reversible protonation of the iron hydride catalyst.

General method for iron-catalyzed multicomponent radical cascades-cross-couplings

[Figure: see text].

Nickel-catalyzed C–P cross-coupling of (het)aryl tosylates with secondary phosphine oxides

A novel and convenient approach to the synthesis of various tertiary phosphine oxides via nickel-catalyzed cross-coupling of (het)aromatic tosylates with secondary phosphine oxides is developed. The reaction employs cheap nickel as the catalyst, 1-(2-(di-tert-butylphosphanyl)phenyl)-4-methoxypiperidine (L3) as the ligand, and pyridine as the base. This reaction produces the corresponding (het)aromatic phosphorus compounds in good to high yields. Moreover, four new tertiary phosphine oxides are reported in this process.

Visible-Light-Induced Ni-Catalyzed Radical Borylation of Chloroarenes

A highly selective and general photoinduced C-Cl borylation protocol that employs [Ni(IMes)₂] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical borylation of chloroarenes is reported. This photoinduced system operates with visible light (400 nm) and achieves borylation of a wide range of chloroarenes with B₂pin₂ at room temperature in excellent yields and with high selectivity, thereby demonstrating its broad utility and functional group tolerance. Mechanistic investigations suggest that the borylation reactions proceed via a radical process. EPR studies demonstrate that [Ni(IMes)₂] undergoes very fast chlorine atom abstraction from aryl chlorides to give [Ni^I(IMes)₂Cl] and aryl radicals. Control experiments indicate that light promotes the reaction of [Ni^I(IMes)₂Cl] with aryl chlorides generating additional aryl radicals and [Ni^II(IMes)₂Cl₂]. The aryl radicals react with an anionic sp²-sp³ diborane [B₂pin₂(OMe)]^- formed from B₂pin₂ and KOMe to yield the corresponding borylation product and the [Bpin(OMe)]^•- radical anion, which reduces [Ni^II(IMes)₂Cl₂] under irradiation to regenerate [Ni^I(IMes)₂Cl] and [Ni(IMes)₂] for the next catalytic cycle.

Study and modular synthesis of unsymmetrical bis(phosphino)pyrrole ligands

We report the facile and modular synthesis of unsymmetrical 1,2-bis(phosphino)pyrrole ligands and their coordination chemistry. These ligands offer a promising alternative to their 1,2-bis(phosphino)benzene congeners, retaining a similar steric profile with attenuated electron donation. Proof-of-principle application of a bis(phosphino)pyrrole ligand in a nickel-catalyzed C-N cross-coupling reaction under mild conditions is demonstrated.

Regio- and stereoselective synthesis of 1,4-enynes by iron-catalysed Suzuki-Miyaura coupling of propargyl electrophiles under ligand-free conditions

The first iron-catalysed cross coupling of propargyl electrophiles with lithium alkenylborates has been developed. Various propargyl electrophiles can be cross-coupled with lithium (E)- or (Z)-alkenylborates in a stereospecific manner to afford the corresponding 1,4-enynes in good to excellent yields. The reaction features high SN2-type regioselectivity and functional group compatibility.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

Rocha MV, Hamlin TA, Poater J, Bickelhaupt FM. Understanding the differences between iron and palladium in cross-coupling reactions

We aim at developing design principles, based on quantum chemical analyses, for a novel type of iron-based catalysts that mimic the behavior of their well-known palladium analogs in the bond activation step of cross coupling reactions. To this end, we have systematically explored C-X bond activation via oxidative addition of CH3X substrates (X = H, Cl, CH3) to model catalysts mFe(CO)4q (q = 0, -2; m = singlet, triplet) and, for comparison, Pd(PH3)2 and Pd(CO)2, using relativistic density functional theory at the ZORA-OPBE/TZ2P level. We find that the neutral singlet iron catalyst 1Fe(CO)4 activates all three C-X bonds via barriers that are lower than those for Pd(PH3)2 and Pd(CO)2. This is a direct consequence of the capability of the iron complex to engage not only in π-backdonation, but also in comparably strong σ-donation. Interestingly, whereas the palladium complexes favor C-Cl activation, 1Fe(CO)4 shows a strong preference for activating the C-H bond, with a barrier as low as 10.4 kcal mol-1. Our results suggest a high potential for iron to feature in palladium-type cross-coupling reactions.

C-Halide bond cleavage by a two-coordinate iron(i) complex

The two-coordinate iron(i) complex [Fe^I(N(SiMe₃)₂)₂]⁻ is highly efficient in the cleavage of C-halide bonds.

Iron-catalysed enantioselective Suzuki–Miyaura coupling of racemic alkyl bromides

The first iron-catalyzed enantioselective Suzuki–Miyaura coupling reaction has been established by using electron-deficient P-chiral bisphosphine ligand (R,R)-QuinoxP*.

Synthesis and Computational Study of a Pyridylcarbene Fe(II) Complex: Unexpected Effects of fac/ mer Isomerism in Metal-to-Ligand Triplet Potential Energy Surfaces

The synthesis and the steady-state absorption spectrum of a new pyridine-imidazolylidene Fe(II) complex (Fe-NHC) are presented. A detailed mechanism of the triplet metal-to-ligand charge-transfer states decay is provided on the basis of minimum energy path (MEP) calculations used to connect the lowest-lying singlet, triplet, and quintet state minima. The competition between the different decay pathways involved in the photoresponse is assessed by analyzing the shapes of the obtained potential energy surfaces. A qualitative difference between facial ( fac) and meridional ( mer) isomers' potential energy surface (PES) topologies is evidenced for the first time in iron-based complexes. Indeed, the mer complex shows a steeper triplet path toward the corresponding ³MC minimum, which lies at a lower energy as compared to the fac isomer, thus pointing to a faster triplet decay of the former. Furthermore, while a major role of the metal-centered quintet state population from the triplet ³MC region is excluded, we identify the enlargement of iron-nitrogen bonds as the main normal modes driving the excited-state decay.

Assessment of the ground spin state of iron(i) complexes: insights from DFT predictive models

Factors governing the ground spin state of iron(i) complexes are analyzed by DFT methods. An efficient benchmarking procedure is reported.

Cobalt-Catalyzed Suzuki Biaryl Coupling of Aryl Halides

Readily accessed cobalt pre-catalysts with N-heterocyclic carbene ligands catalyze the Suzuki cross-coupling of aryl chlorides and bromides with alkyllithium-activated arylboronic pinacolate esters. Preliminary mechanistic studies indicate that the cobalt species is reduced to Co0 during the reaction.

A Zinc Catalyzed C(sp3 )-C(sp2 ) Suzuki-Miyaura Cross-Coupling Reaction Mediated by Aryl-Zincates

The Suzuki-Miyaura (SM) reaction is one of the most important methods for C-C bond formation in chemical synthesis. In this communication, we show for the first time that the low toxicity, inexpensive element zinc is able to catalyze SM reactions. The cross-coupling of benzyl bromides with aryl borates is catalyzed by ZnBr2 , in a process that is free from added ligand, and is compatible with a range of functionalized benzyl bromides and arylboronic acid pinacol esters. Initial mechanistic investigations indicate that the selective in situ formation of triaryl zincates is crucial to promote selective cross-coupling reactivity, which is facilitated by employing an arylborate of optimal nucleophilicity.

A Physical-Inorganic Approach for the Elucidation of Active Iron Species and Mechanism in Iron-Catalyzed Cross-Coupling

Detailed studies of iron speciation and mechanism in iron-catalyzed cross-coupling reactions are critical for providing the necessary fundamental insight to drive new reaction development. However, such insight is challenging to obtain due to the prevalence of mixtures of unstable, paramagnetic organoiron species that can form in this chemistry. A physical-inorganic research approach combining freeze-trapped inorganic spectroscopic studies, organometallic synthesis and GC/kinetic studies provides a powerful method for studying such systems. Mössbauer, EPR and MCD spectroscopy enable the direct investigation of in situ formed iron species and, combined with GC analysis, the direct correlation of reactions of specific iron species to the generation of organic products. This review focuses on a description of the key methods involved in this physical-inorganic approach, as well as examples of its application to investigations of iron-SciOPP catalyzed cross-coupling catalysis.

Intermediates and Reactivity in Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl Halides

Iron-catalyzed cross-coupling reactions using alkynyl nucleophiles represent an attractive approach for the incorporation of alkynyl moieties into organic molecules. In the present study, a multitechnique approach combining inorganic spectroscopic methods, inorganic synthesis, and reaction studies is applied to iron-SciOPP catalyzed alkynyl-alkyl cross-couplings, providing the first detailed insight into the effects of variation from sp2- to sp-hybridized nucleophiles on iron speciation and reactivity. Reaction studies demonstrate that reaction of FeBr2(SciOPP) with 1 equiv (triisopropylsilyl)ethynylmagnesium bromide (TIPS-CC-MgBr) leads to a distribution of mono-, bis-, and tris-alkynylated iron(II)-SciOPP species due to rapid alkynyl ligand redistribution. While solvents such as THF promote these complex redistribution pathways, nonpolar solvents such as toluene enable increased stabilization of these iron species and further enabled assessment of their reactivity with electrophile. While the tris-alkynylated iron(II)-SciOPP species was found to be unreactive with the cycloheptyl bromide electrophile over the average turnover time of catalysis, the in situ formed neutral mono- and bis-alkynylated iron(II)-SciOPP complexes are consumed upon reaction with the electrophile with concomitant generation of cross-coupled product at catalytically relevant rates, indicating the ability of one or both of these species to react selectively with the electrophile. The nature of the reaction solvent and Grignard reagent addition rate were found to have broader implications in overall reaction selectivity, reaction rate, and accessibility of off-cycle iron(I)-SciOPP species. Additionally, the effects of steric substitution of the alkynyl Grignard reagent on catalytic performance were investigated. Fundamental insight into iron speciation and reactivity with alkynyl nucleophiles reported herein provides an essential foundation for the continued development of this important class of reactions.

Convenient and General Zinc-Catalyzed Borylation of Aryl Diazonium Salts and Aryltriazenes under Mild Conditions

A convenient and general zinc‐catalyzed borylation of aryl diazonium salts and aryltriazenes has been developed. With bis‐ (pinacolato)diboron as the borylation reagent, aryldiazonium tetrafluoroborate salts and aryltriazenes were transformed into the corresponding arylboronates in moderate to excellent yields under mild conditions. As a convenient and practical methodology, no additional ligands, base, or any other additives are required here.